A flash memory and a ferroelectric memory are well-known as nonvolatile memories capable of retaining stored information even after power is turned off.
Among these nonvolatile memories, the flash memory includes a floating gate embedded in a gate insulating film of an insulated-gate field-effect transistor (IGFET), and stores information by accumulating, in this floating gate, electric charges indicating information to be stored. However, since a tunnel current needs to pass through the gate insulating film at the time of writing and erasing the information, the flash memory has a drawback that a relatively high voltage is needed.
In contrast, the ferroelectric memory, which is also referred to as a ferroelectric random access memory (FeRAM), stores information by utilizing the hysteresis characteristic of a ferroelectric film provided to a ferroelectric capacitor. The ferroelectric film is polarized in response to a voltage applied between upper and lower electrodes of the capacitor, and spontaneous polarization remains even after the voltage is removed. When the polarity of the applied voltage is reversed, the spontaneous polarization is also reversed. By causing directions of the spontaneous polarization to correspond respectively to “1” and “0”, the information is written in the ferroelectric film. Advantages of the FeRAM are that the voltage required for writing is lower than that required for writing in the flash memory, and that information can be written at a higher speed than that of the flash memory.
In the above-described FeRAM, a metal wiring is formed on an interlayer insulating film for applying a voltage to upper and lower electrodes of the capacitor. Examples of structures of metal wirings are shown in Japanese Patent Application Publication Nos. 8-330417, 11-274297, 2002-343857, 11-8299, 10-209277, 8-293549, 8-250589, 11-186382, 8-330422 (hereinafter referred to simply as Patent Documents 1 to 9, respectively).
Of these patent documents, in the patent documents 1 to 7, sidewalls are formed on sides of a wiring so that a margin for alignment of a contact hole and a wiring is made larger.
In addition, in the patent documents 8 and 9, insulating sidewalls are formed on sides of a wiring so as to prevent a conductive plug from increasing contact resistance even if the conductive plug on the wiring is misaligned.